Measurements of the large-scale DC Earth potential and possible implications for the geomagnetic dynamo.

Results are reported on the magnitude of the large-scale DC Earth potential using measurements made on a section of a recently laid transatlantic telecommunications cable. Analysis of the data acquired on the 4476 km cable yields a mean DC potential drop of =0.08 mV/km. Interpreted in terms of a generation of the potential by the Earth's geodynamo, such a small value of the mean potential implies, under certain assumptions, that the toroidal and poloidal magnetic fields of the dynamo are approximately equal at the coremantle boundary.